Project Summary A novel technique called transcranial current stimulation (TCS) creates small electrical fields in the brain through electrodes placed on the scalp. As a method for neuromodulation, TCS carries with it many practical benefits: it is portable (battery-operated), inexpensive, and easily deployable in the clinic and at home. Due to this simplicity and apparent versatility there has been an explosion in the number of studies currently underway using either direct or alternating transcranial currents (over 500 clinical trials are listed with clinicaltrials.gov). Despite this ubiquitous use of the technique in clinical and basic research, there is substantial uncertainly as to its mechanism of action, and even its basic dosage/response relationships are poorly understood. This project will use intracranial recordings in the primary visual cortex of nonhuman primates to understand how TCS changes neural activity. The first aim is to understand the parameters (e.g. current strength, electrode montage, duration) that affect the ability of direct currents to modulate neural excitability (tDCS). The second aim is to understand the parameters that affect the neuromodulatory efficacy of transcranial random noise currents (tRNS). The third aim is to understand the parameters that affect the efficacy of alternating currents (tACS).